Displacement measuring interferometers (“DMIs”) are well known in the art, and have been used to measure small displacements and lengths to high levels of accuracy and resolution for several decades. Among such devices, helium-neon displacement measuring laser interferometers have enjoyed relatively widespread application owing to their high degree of stability and monochromaticity. Interferometers require careful alignment of mirrors that must be sustained over extended periods of time, however, which can present considerable practical difficulties.
A double-pass interferometer may be rendered partially insensitive to mirror misalignments by double-passing each arm of the interferometer and incorporating a means of inverting the wavefronts between passes. See, for example, “A Double-Passed Michelson Interferometer” by S. J. Bennett in Optics Communications, Volume 4, number 6, February/March, 1972, where double-passing is achieved using a polarized beam-splitter, two quarter-wave plates and a cube-corner reflector that serves as an inverting component. The entirety of the foregoing paper by Bennett is hereby incorporated by reference herein. In consequence of their commercial viability, robustness, stability and accuracy, double-pass displacement measuring interferometers find relatively common use in high accuracy displacement measurements.
Despite the many advances that have been made in the field of DMIs generally, however, measurement errors and inaccuracies persist. Among the factors contributing to such errors and inaccuracies are alignment errors and path length errors, optical mixing, thermal effects, polarization leakage (or the unintended mixing of measurement and reference beams), diffraction-induced fringing, non-linear relationships between phase and displacement, and other errors. See, for example, “Recent Advances in Displacement Measuring Interferometry” by Norman Brobroff in Meas. Sci. Technol. 4 (1993) 907-926, and “An Investigation of Two Unexplored Periodic Source Errors in Differential-Path Interferometry” by Schmitz and Beckwith in Precision Engineering 27 (2003) 311-322, where some of these factors are discussed in detail. The respective entireties of the foregoing papers by Broboff and Schmitz et al. are hereby incorporated by reference herein.
Most DMIs in the prior art combine reference and measurement beams before they are presented to the optical portion of an interferometer system. The non-ideal characteristics of the source and optics result in mixing of the reference and measurement beams before the desired displacement is measured. This is one of the principal means by which non-linear errors are introduced in DMIs. Another principal source of non-linear error in DMIs is diffraction-based interference. Some prior art DMIs employ a reflective aperture to separate a reference beam from a measurement beam, the two beams sharing a common annulus up to the reflective aperture. The result of such an architecture is that an interference beam is formed, which can degrade performance.
In a paper presented at the Annual Meeting of the ASPE in 2001 entitled “Demonstration of Sub-Angstrom Cyclic Non-Linearity Using Wavefront-Division Sampling with a Common-Path Laser heterodyne Interferometer,” Feng Zhao of the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif. discloses a common-path heterodyne interferometer that attempts to minimize non-linear errors. Zhao isolates the reference and measurement beams over most of the optical path to the detector by using separate fiber optic channels for the reference and measurement beams. The reference beam has a first frequency and the measurement beam has a second frequency different from the first frequency. In rough terms, first and second beams corresponding to the first and second frequencies are generated and measured at separate first and second detectors. In some systems, however, things are more complicated than this. Instead, so-called “local oscillator” and “probe” beams are emitted by the source as two separate beams, the beams being mixed in the interferometer to produce measurement and reference beams at the output. This topic is discussed in further detail below.
For purposes of clarity and to avoid confusion, the terms “measurement beam” and “reference beam” are employed herein, but are to be understood as potentially being interchangeable with the terms “local oscillator beam” and “probe beam,” respectively, depending upon the particular context in which either term may appear.
The first beam may be referred to as a reference beam produced by means of the first frequency beam impinging upon a stationary aperture. The second beam may be referred to as a measurement beam produced by the second frequency beam impinging upon a moving target. The phase difference between the first and second beams represents the position of the target. Zhao's interferometer architecture reduces non-linear errors in measured displacements. Zhao employs a wavefront division scheme, however, in which diffraction-based interference errors remain important because measurement and reference beams are annularly spaced apart from one another over essentially the same optical path. Moreover, it is not clear how Zhao's approach could be expanded to more than one optical axis.
What is needed is a DMI that further minimizes non-linear and diffraction-induced errors, and that may be scaled up or down over multiple optical axes in a straightforward and economic manner.